1. Field of the Invention
The present invention relates to a process for carrying out multiphase reactions in a tubular reactor, in particular for preparing aldehydes by reacting olefinically unsaturated compounds with hydrogen and carbon monoxide in the presence of a catalyst.
2. Discussion of the Background
Aldehydes are used in the synthesis of many organic compounds. Their direct secondary products are alcohol and carboxylic acids which are used industrially. The alcohols prepared from the aldehydes are used, inter alia, as solvents and as a precursor for the preparation of plasticizers and detergents.
It is known to prepare aldehydes and alcohols by reacting olefins with carbon monoxide and hydrogen. The reaction is catalyzed by hydrido metal carbonyls, preferably those of metals of group 8 of the Periodic Table of the Elements. In addition to cobalt, which is used industrially to a broad extent as catalyst metal, rhodium has recently achieved increasing importance. In contrast to cobalt, rhodium permits the reaction to be carried out at relatively low pressure. The hydrogenation of the olefins to give saturated hydrocarbons is markedly less when rhodium catalysts are used than when cobalt catalysts are used.
In the case of the processes carried out industrially, the rhodium catalyst is formed during the process from a catalyst precursor, synthesis gas and, if appropriate, other ligands having a modifying action. In the case of use of modified catalysts, the modifying ligands can be present in the reaction mixture in excess. Ligands which have proved particularly expedient are tertiary phosphines or phosphates. Their use enables the reaction pressure to be decreased to values below 300 bar.
However, in the case of the above process, problems arise in separating off the reaction products and recovering the catalysts which are homogeneously dissolved in the reaction product. Generally, for this purpose, the reaction product is distilled off from the reaction, mixture. In practice, this route, because of the thermal sensitivity of the catalyst or the products formed, can only be taken in the event of the hydroformylation of lower olefins having up to about 5 carbon atoms in the molecule.
On an industrial scale, C4 and C5 aldehydes are prepared, for example in accordance with DE 3234701 or DE 2715685.
In the latter process, the catalyst is dissolved in the organic phase which is composed of product and high-boilers (resulting from the product). Olefin and synthesis gas are introduced into this mixture. The product is discharged from the reaction together with the synthesis gas, and in a more recent variant taken off as liquid. Since the catalyst slowly decays in its output, a portion must continuously be discharged together with high-boilers and replaced by an equivalent amount. Because of the high cost of rhodium, recovering the rhodium from the discharge stream is essential. The workup process is complex and thus burdens the process.
According to DE 3234701, this disadvantage is overcome, for example, by the catalyst being dissolved in water. The water solubility of the rhodium catalyst used is achieved by using trisulfonated triarylphosphines as ligands. Olefin and synthesis gas are passed into the aqueous phase. The product resulting from the reaction forms a second liquid phase. The liquid phases are separated outside the reactor and the catalyst phase separated off is recycled to the reactor.
Technically, the above two processes are multiphase reactions.
Multiphase reactions are taken below to mean reactions that proceed with the participation of two or more immiscible, or only partially miscible, fluid phases. This includes, for example, reactions between a gas phase and a liquid phase (gl), between two liquid phases which are immiscible or have a miscibility gap (ll) and reactions in which both two liquid immiscible, or only partially miscible, phases and one gas phase participate (gll).
However, in addition, the use of other fluid phases, e.g. supercritical phases, is also conceivable. A supercritical phase of this type can occur alternatively to said phases, but also additionally.
Examples of industrially important gas-liquid reactions (gl) are, in addition to the hydroformylation of liquid olefins in the presence of a catalyst dissolved in organic phase, the reaction of acetylene with carboxylic acids or hydrogenations with homogeneously dissolved catalysts or oxidations with air or oxygen.
The above-noted examples share the problem of mass transfer, since the reaction partners are present in different phases. In the case of hydroformylation in the 3-phase system, the process procedure is particularly difficult, since the starting materials are present in three separate phases.
Not only olefin but also synthesis gas must be transported into the aqueous catalyst phase in order to come into contact there with the catalyst. Finally, backtransport out of the aqueous phase must take place. Since the transport processes are frequently slower than the actual reaction, such reactions are determined by the rate of mass transfer, and one speaks of a transport-inhibited reaction.
Multiphase reactions are associated with a series of problems which makes them considerably more difficult to carry out industrially than is the case with simple homogeneous reactions. Some typical problems are mentioned below:
In all cases, the substances must be brought into contact with one another as intimately as possible in order to minimize the problem of mass transfer: a mass transfer area as as large as possible must be generated between the phases. On the other hand, the phases must be able to be easily separated again after reaction is completed. Too intensive a mixing can lead to problems here. When two liquid phases are present, emulsion formation can occur, and in the case of gas-liquid processes foaming can occur. In the 3-phase process mentioned, all problems can even occur simultaneously.
In addition to a high mass transfer surface area, as, as high as possible a mass transfer coefficient k1 should be achieved in all multiphase reactions. Overall, what is termed the KLA value, i.e. the product of k1 and as, in the mass transfer equation
j=kl*as*(C*xe2x88x92C)
where
j (mol/s) is the molar flow rate of the reacting component passing through the phase interface,
kl (m/s) is the mass transfer coefficient,
as (m2) is the phase interface area in the reactor,
C* (mol/m3) is the maximum solubility of the starting material in the second phase, and
C (mol/m3) is the actual concentration of the starting material, which in turn is coupled to the reaction rate, should be a maximum.
Another problem in the case of multiphase reactions is heat dissipation with exothermic reactions. If the reaction rate is successfully increased by improving the mass transfer, obviously, more heat must be dissipated, which can lead to an unwanted temperature increase up to reaction runaway.
These problems of the multiphase reaction can be solved technically, e.g. by using a stirred-tank reactor.
For the hydroformylation of olefins, the use of a stirrer under elevated pressure is disadvantageous, since in this case a fault-susceptible shaft seal must be provided. In addition, in this process, a plurality of stirrers must be used in order to achieve adequate phase mixing.
The high heat of reaction produced in hydroformylations can generally only be controlled by using heat exchangers mounted in the interior of the reactor.
The use of a stirred tank always leads to back-mixing, which decreases the effective concentration of the reactants, which leads to lowering the space-time yield. This disadvantage must be paid for by the capital expenditure on expensive reaction space.
During the hydroformylation of olefins, the space-time yield decreases sharply anyway with increasing number of carbon atoms in the olefin. The correlation between molecular size and reaction rate is known (B. Cornils, W. Herrmann, Aqueous-Phase Organometallic Catalysis, Concepts and Application, Verlag Wiley-VCH, pp. 308-310). Thus, for example, the ratio of hydroformylation reaction rates of 1-pentene to 1-hexene is 2.6/1. A conversion of higher olefins therefore becomes increasingly uneconomic.
In view of the above considerations, there is a need for a process that avoids the abovementioned disadvantages and which, in addition, can be implemented industrially in a simple manner.
It is an object of the present invention to provide a process for carrying out multiphase reactions which is especially suitable for preparing aldehydes by means of catalytic hydroformylation of olefins.
This object and other objects have been achieved by the present invention, which provides a process that includes carrying out a multiphase reaction catalytically in a tubular reactor, the multiphase reaction including a continuous phase and at least one disperse phase, wherein a catalyst is present in the continuous phase and at least one starting material is present in the disperse phase, and wherein the tubular reactor includes a loading factor, B, that is equal to or greater than 0.8.